Mechanical systems such as internal combustion engines and automatic transmissions typically include a lubrication pump to provide lubricating oil, under pressure, to many of the moving components and/or subsystems of the mechanical systems. In most cases, the lubrication pump is driven by a mechanical linkage to the mechanical system and thus the operating speed, and output, of the pump varies with the operating speed of the mechanical system. While the lubrication requirements of the mechanical system also vary with the operating speed of the mechanical system, unfortunately the variation in the output of the pump and the variation of the lubrication requirements of the mechanical system are generally nonlinear. The difference in these requirements is further exacerbated when temperature related variations in the viscosity and other characteristics of the lubricating oil are factored in.
To deal with these differences, prior art fixed displacement lubricating pumps were generally designed to operate effectively at high, or maximum, oil temperatures, resulting in an oversupply of lubricating oil at most mechanical system operating conditions and a waste, or pressure relief, valve was provided to “waste” the surplus lubricating oil back into the pump inlet or oil sump to avoid over pressure conditions in the mechanical system. In some operating conditions such as low oil temperatures, the overproduction of pressurized lubricating oil can be 500% of the mechanical system's needs so, while such systems work reasonably well, they do result in a significant energy loss as energy is used to pressurize the unneeded lubricating oil which is “wasted” through the relief valve.
More recently, variable displacement vane pumps have been employed as lubrication oil pumps. Such pumps generally include a control ring, or other mechanism, which can be moved to alter the displacement of the pump and thus its output at an operating speed. Typically, a feedback mechanism, in the form of a piston or control chamber supplied with pressurized lubricating oil from the output of the pump, either directly or via an oil gallery in the mechanical system, alters the displacement of the pump to avoid over pressure situations in the engine over the expected range of operating conditions of the mechanical system. An example of such a pump is shown in U.S. Pat. No. 4,342,545 to Schuster.
While such variable displacement pumps provide some improvements in energy efficiency over fixed displacement pumps, they still result in a significant energy loss as they must still be designed to provide oil pressures which meet the highest expected mechanical system requirements and operating temperatures, even when the mechanical system operating conditions normally do not necessitate such high requirements.